A known thermal spraying lining process for piston rings employs 99.9% pure molybdenum feeding wires, which are melted by respective torches of oxygen and acetylene. In this process, droplets of melted molybdenum are sprayed against the external contact faces of piston rings, said faces presenting a relatively low temperature as compared to that of the droplets upon reaching said external contact face, compressed air being used as a propellant. The adhesion between the droplets and the external contact face of the piston rings is mechanically achieved, by the solidification of said droplets onto a somewhat roughened surface of the external contact face of the rings, due to the high contraction tension during solidification.
Besides the high cost of this process, which uses only molybdenum, the resultant lining layer proves to be fragile, due to the fact that the welding of successive lining layers occurs through oxide-rich interfaces. Such fragility makes the lining susceptible to cracks and chips, when the piston ring is submitted to mechanical stresses. The integrity of the lining with molybdenum is affected by the operational high temperatures of said piston ring close to the combustion chamber, mainly due to the substantial difference between the thermal expansion coefficients of the molybdenum and the basic metal of iron alloy, giving rise to said cracks when under stress.
Moreover, the spraying lining process using wires is not very versatile in relation to the combination of different materials.
Another known process employs powder spraying, which enables the association of materials, but has a very high cost, mainly in the case of plasma arc spraying.